In the manufacture of aircraft skin panels, it is common to attach I-section stringers to the back of the skin panels. These stringers not only provide reinforcement, they also provide a location for attaching the skin panels to the ribs or frame of the aircraft.
Typically, the skin panels and I-section stringers are made from a graphite epoxy composite material which must be cured. One conventional method has been to bond uncured I-section stringers to uncured skin panels in a single cocuring operation. In this manner the stringers are bonded to the skin panels during the curing process. Another method has been to cure the skin panels first, and then to bond the precured skin panels to uncured stringers during a second curing operation. Still another method has been to cure the stringers and skin panels separately and then bond them with adhesive in a subsequent elevated temperature operation.
Conventionally, the curing of composite materials has involved an autoclave process which is quite labor intensive and costly. Prior to placing the stringers in the autoclave, preparation of the stringers has involved numerous manually performed tasks such as, for example, carefully trimming stringer flanges flush with layup mandrel surfaces to avoid mandrel entrapment during cure, taping the ends and sides of the stringer/mandrel assemblies to contain epoxy resin flow, placing a parting film over the components, covering the components with a breather material, placing a nylon bag over the breather material and taping the bag in place. Once the stringers have been cured, this covering material must be removed.
In order to minimize the number of times the above autoclave process is performed, it has been preferable to use a single cocuring process. However, the difficulty of fitting bag materials snugly into the corner where the stringer/mandrel assemblies join the skin panel can result in bridging of the bag and a consequent deformation of the skin panels on the bagside surface during cure. The deformation problem can be avoided by precuring the skin panels, however this involves utilizing the autoclave a first time to precure the skin panels, and a second time to bond uncured stringers to the skin panels. In this case of precured stringers bonded to precured skins, one or more autoclave cycles are required to produce the precured details, followed by a secondary bonding operation in an autoclave or oven.
It was perceived that the fabrication and cost problems of conventional methods could be solved by a cost-effective, non-autoclave process for producing precured stringers.